The invention relates to a batteryless transponder which, in its reception phase, acquires its supply energy by the rectification of an RF interrogation pulse transmitted by an interrogation device and then stores it in a storage element serving as a supply voltage source during the transmission phase, and which, in response to the reception of the RF interrogation pulse, transmits information stored in it, wherein the coil of a tuned circuit serves as an antenna for both the reception of the interrogation pulse and the transmission of the information stored.
A transponder of this kind is known from EP 0 301 127 B1. When this known transponder has received an RF interrogation signal via its tuned circuit, which by rectification caused the charging of a capacitor, serving as a storage element, a supply voltage for the transponder then becomes available at this capacitor. The transponder recognises the termination of the interrogation pulse and causes a switch to close, by which the voltage being available at the capacitor is applied to the tuned circuit of the transponder. The closure of the switch is only of short duration, so that energy is supplied to the tuned circuit for the purpose of inducing resonant oscillations. Once the switch is opened, oscillations in the tuned circuit continue at an aperiodically decreasing amplitude. In order to maintain the resonant oscillations, the switch is again briefly closed after a predetermined number of oscillation periods, such as eight oscillation periods, for the purpose of supplying new energy to the tuned circuit and so to maintain the oscillations. This can be continued to the point when there is no longer sufficient energy available in the capacitor serving as the storage element to maintain oscillations. The circuit arrangement for the supply of energy to the tuned circuit is realised in such a way that, when the switch closes, the tuned circuit is connected directly in parallel to the storage element. The consequence of this is that the peak voltage of the resonant oscillations can never assume more than double the value of the supply voltage available at the storage element. The maximum field strength that can be reached when transmitting the information stored in the transponder, a process initiated by the induced resonant oscillations, is therefore limited, which, of necessity, also leads to a limitation of the reception range within which the information transmitted can still be received with a suitable receiver.
The invention enables a batteryless transponder of the type mentioned above obtain a higher field strength, and therefore a greater range for the information transmitted.
According to an embodiment of the invention, this improvement is achieved by providing a controllable switching device, which disconnects the tuned circuit from the supply voltage source during the transmission phase, and connects it to the supply voltage source only for a duration which is short as compared with a quarter period of the resonant oscillations, depending on the occurrence of each oscillation minimum of the resonant oscillations.
In the case of the transponder according to an embodiment of the invention, the tuned circuit is no longer hard-connected to the supply source during the transmission phase, so that the amplitude of its oscillations is not limited to a value double that of the supply voltage. Instead, after having been supplied with energy by a brief connection with the supply source, the tuned circuit is allowed to oscillate freely, so that an oscillation amplitude greater than double the supply voltage results, on account of the Q-factor. As a result of this increased oscillation amplitude, the desired reception range of the signal transmitted by the tuned circuit is correspondingly enhanced.